FIELD OF THE INVENTION
The invention relates to a process for the production of a screen plate for the foot of a fuel assembly, in which a cross section of the plate is matched to a cross section of the foot and screen apertures are matched to dimensions of foreign bodies to be retained. The invention also relates to a fuel assembly with a bundle of fuel rods disposed regularly next to one another in rows and columns between a head and a foot of the assembly. The fuel assembly may also have supporting structural elements, such as guide tubes or supporting fuel rods, a screen plate in the foot which may support at least some of the structural elements, extend virtually over the cross section of the bundle and be made in one piece or of plate parts disposed next to one another, and passage apertures leading from a lower to an upper surface of the screen plate for a coolant stream, the passage apertures having dimensions selected according to dimensions of foreign bodies to be screened out from the coolant stream and possibly having cross sections with corners, as in U.S. Pat. No. 5,030,412. The geometry of the holes and passage apertures guiding the coolant stream is worked into the one-piece plate by such a non-mechanical process for the erosion of material.
It is not possible to completely exclude the possibility that under particularly unfavorable circumstances foreign bodies will enter the coolant stream which is guided through the core of a nuclear reactor in order to cool the fuel assemblies. Thus, for example, metal chips originating from manufacture may still be located in the heat exchangers or other reactor components or springs, pieces of wire or other fragments can be broken off while the reactor is in operation. Such foreign bodies (or "debris") can be flushed by the coolant stream to the meshes of the spacers, in which the fuel rods within the fuel assembly are fixed. Strong vibrations, to which the fuel rods are exposed in the coolant stream, then give rise to friction (or "fretting") and damage which can lead to the destruction of fuel rods.
Accordingly, there has recently been the requirement to trap such foreign bodies by means of an appropriate screen in the fuel assembly foot through which the coolant flows.
It is proposed in U.S. Pat. No. 4,832,905 to construct a rod holder as a grid being formed of a large number of intersecting webs. That results in fine grid meshes which screen out the troublesome particles from the coolant stream flowing through. The stability requirements demand a considerable overall height of the grid, complicated welding seams between the webs of the grid and special fastenings of the supporting rods.
It is more often proposed to achieve the necessary screening-out of the foreign bodies in the fuel assembly foot by means of a specific screen disposed above or below the rod-holding plate. According to U.S. Pat. No. 4,678,627, a basket made from perforated sheet metal is mounted as a screen in the fuel assembly foot. According to U.S. Pat. No. 4,684,495, the screen is constructed as a coarse-mesh grid, but adjacent grid meshes are connected to one another by means of apertures in the grid webs and the grid webs carry straps which block the direct flow path through the grid meshes, so that the coolant stream is conveyed through the grid along winding paths, at the turns of which foreign bodies are trapped.
U.S. Pat. No. 5,030,412 describes a grid-like screen plate with horizontal longitudinal webs which are bent in a wave like manner in the (vertical) direction of the coolant stream, so that a rectilinear path through the screen plate is blocked. Those longitudinal webs are welded to transverse rods to form a grid having rectangular meshes. The screen plate is disposed underneath the rod-holding plate, but can also be produced together with the rod-holding plate as an integral casting, in which case the wavy longitudinal webs are not welded to transverse rods, but are formed on the lower edge of intersecting ribs which form a grid having elongate rectangular meshes and beyond which the longitudinal webs project on the lower surface of the grid. The ribs and longitudinal webs must be relatively thick and occupy a considerable part of the plate cross section, so that their flow resistance amounts to 50% of the flow resistance of the entire fuel assembly.
Such special constructions involve a very high outlay both in terms of production and with regard to the space requirement in the fuel assembly foot.